Method and apparatus for dynamic nitriding

ABSTRACT

A nitriding method includes the steps of nitriding an object held under a nitriding gas atmosphere in a sealed furnace and applying vibration energy to one or both of the nitriding gas and the object W to facilitate nitriding. A nitriding apparatus includes a nitriding furnace for holding an object W to be nitrided in a sealed manner, means  30  for supplying a nitriding gas to the furnace, and means  2  for applying vibration to the atmosphere gas in the furnace  1  to faciliate nitriding. According to the method or apparatus, nitriding-resistant or complex-shaped materials can be nitrided at high efficiency and a nitrided layer can be formed at a lower temperature for a shorter time as compared with conventional processes.

FIELD OF THE INVENTION

[0001] The present invention relates to a nitriding method that can provide dramatically enhanced nitriding efficiency in gas nitriding processes of such materials as steel members and nitriding-resistant materials, and an apparatus for use therein.

DESCRIPTION OF THE RELATED ART

[0002] Conventionally, nitriding processes are used to harden the surface of steel members. The nitriding processes are carried out at a relatively low temperature as compared with cementation processes and therefore can provide the nitrided materials with less deformation or distortion. The nitriding processes can also form a very hard nitrided layer in the surface of steels and therefore are widely used as a surface treatment process for providing a good wear or corrosion resistance. Known examples of the nitriding process include a gas nitriding process, a salt bath nitriding process, and an plasma nitriding process.

[0003] Generally, the salt bath nitriding process uses cyanide salts, which can provide a harmful working environment, and requires a high cost for waste disposal. The ion nitriding process, which uses a discharge process under reduced pressure, is suited for the treatment of simple-shaped objects. In the ion nitriding process, however, it would be difficult to evenly nitride objects that have complicated shapes, small holes, or deep holes.

[0004] In the gas nitriding process, the object such as a steel product is heated in a nitrogen-containing nitriding gas such as ammonia gas. The heated nitriding gas is decomposed into nitrogen atoms, which chemically react with the iron components in the steel surface to form a nitrided layer for hardening the steel surface. The gas nitriding process can be carried out in a good working environment and applied to complex-shaped objects, so that it can be free from the problem with the salt bath or ion process.

[0005] The gas nitriding process, however, requires a step of removing a passive state film from the surface of nitriding-resistant materials such as austenitic stainless steels before nitriding. It can also provide complex-shaped materials with uneven thickness of the nitrided layer or form insufficiently nitrided portions at small or deep holes.

[0006] In general, the nitriding gas is superficially brought into contact with the object in the nitriding process so that the chemical reactions involved in the nitriding are slow and the process for a thick nitrided layer needs a long time (at least 40 hours) or a high temperature treatment (550 to 580° C. or above). Such a long time or high temperature process can reduce the hardness of the nitrided layer, increase the embrittled layer (white layer), or increase the dimensional change, and adversely affect the metallurgical properties of the object. The long time process can also increase the usage of the nitriding gas, decrease the productivity of the nitrided product, or provide the product with low cost-performance.

SUMMARY OF THE INVENTION

[0007] In light of the above-mentioned problems, the present invention is directed to a new epoch-making nitriding method that can nitride nitriding-resistant materials or complex-shaped materials with good efficiency and form a excellent nitrided layer at a low temperature for a short time period in contrast to conventional gas nitriding processes, and an apparatus for use therein.

[0008] According to the present invention, it provides a nitriding method in which a nitriding reaction of an object is allowed to proceed under vibrating conditions. Such a method includes the steps of nitriding the object held under an atmosphere gas in a sealed nitriding furnace and applying vibration energy to one or both of a nitriding gas and the object to facilitate nitriding.

[0009] The present invention is also directed to an apparatus including means for applying vibration to one or both of gas and an object to be nitrided.

[0010] In a general gas nitriding process, the surface of steel is heated and brought into contact with a nitriding gas such as ammonia gas. By the catalysis of the steel surface, the ammonia gas is decomposed into active atomic nitrogen, which reacts with the iron components in the steel to form a nitrided layer.

[0011] For example, by the nitriding reaction of iron (Fe) with ammonia (NH₃), a nitrided layer ([Fe]N layer) is formed in the surface of steel according to the following formula:

NH₃+[Fe]→H₂+[Fe]N   (1)

[0012] As a nitrided iron, there are Fe₂N and Fe₄N.

[0013] On the steel containing an alloy such as aluminum (Al), chromium (Cr), titanium (Ti), and vanadium (V) alloys, the activated nitrogen atoms react with the alloy element to form aluminum nitride (AlN), chromium nitride (Cr₂N₃), or the like according to the following formulas:

NH₃+Al→H₂+AlN   (2)

NH₃+Cr→H₂+Cr₂N₃   (3)

[0014] Such materials as iron nitrides (Fe₂N and Fe₄N), aluminum nitride (AlN), and chromium nitride (Cr₂N₃) are insoluble in iron, hard and stable, and have the function of hardening the steel surface.

[0015] The inventor has made active investigations on the gas nitriding process and found that the activity of the atomic nitrogen generated by the contact of the nitriding gas with the steel surface significantly affects the progress of the nitriding reaction. Thus, the inventor has made various experiments to find out a method for facilitating the activation of the nitriding gas. In order to facilitate the activation, the nitriding reaction temperature could be raised, but in such a case, embrittled layers can be formed, or distortion in shape can occur as mentioned above.

[0016] As a result of the experiments and the consideration, the inventor has found that by the contact of the steel surface with a nitriding gas being provided with vibration, the activation of the nitrogen can be dramatically enhanced in the catalytic reaction between the nitriding gas and the steel, and the efficiency in nitriding steels can drastically be improved. In the description, such an improved process is called a dynamic nitriding method.

[0017] The nitrided material obtained by this method is hard and stable and contains fine precipitates dispersed in the alpha iron lattice. As a result, it has been confirmed that the alpha iron lattice is provided with high distortion and the steel is significantly hardened.

[0018] Preferred means for applying vibration to the nitriding gas includes a technique of allowing the nitriding gas to collide against a diaphragm in a sealed nitriding furnace in supplying the nitriding gas to the furnace so that a shock wave is generated in the nitriding gas-containing atmosphere gas in the furnace. A principal part of the technique is a step of producing a dynamic nitriding reaction, in other words, producing a catalytic reaction between the nitriding gas and the steel in such a state that molecular vibration or mechanical vibration is involved, in addition to the conventional fluid catalytic reaction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a side view showing the mechanism of a gas vibration type nitriding apparatus;

[0020]FIGS. 2A and 2B are schematic views showing a first example of the vibration-applying means;

[0021]FIG. 3 is a schematic view showing a second example of the vibration-applying means;

[0022]FIG. 4 is a diagram showing a gas flow;

[0023]FIG. 5 shows hardness of an object (an austenitic stainless steel) with respect to distances from the surface of the object;

[0024]FIG. 6 shows hardness of an object (a martensitic stainless steel) with respect to distances from the surface of the object;

[0025]FIG. 7 shows hardness of an object (a hot work tool steel) with respect to distances from the surface of the object;

[0026]FIG. 8 is a micrograph showing a surface side section of a martensitic stainless steel object nitrided according to the present invention; and

[0027]FIG. 9 is a micrograph showing a surface side section of a martensitic stainless steel object nitrided by a conventional process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Referring to the attached drawings, the present invention is described in detail in the following example.

[0029]FIG. 1 shows an example of the dynamic nitriding method according to the present invention. In this example using a gas vibration type nitriding apparatus, vibration is applied to a nitriding gas being introduced into a nitriding furnace in the step of supplying the nitriding gas to the nitriding furnace, so that shock and vibration is being generated in the atmosphere gas (a mixture of the nitriding gas and an inert gas) in the furnace while the nitriding is allowed to proceed. In the drawing, numeral 1 represents a sealed nitriding furnace in which an object W to be nitrided such as a steel product is placed. On the sides of the furnace 1 are arranged a gas control unit 3 for controlling the feed rate, pressure and the like of the nitriding gas being supplied to the nitriding furnace 1 and a temperature control unit 4 for controlling the temperature of the nitriding furnace 1. Numeral 2 represents vibration-applying means provided according to the present invention, which will be described below in detail.

[0030] The nitriding furnace 1 has a furnace body 10, a retort 11 for holding the object W in the furnace body 10, and a heater 12 for heating the retort 11. The upper portion of the furnace 1 is provided with an opening 13 for pulling out the retort 11. The upper portion of the nitriding furnace 1 is also provided with a nitriding gas supply line 30 for supplying the nitriding gas into the furnace body 10, and an inert gas supply line 31 for supplying an inert gas (e.g. N₂ gas) excluding the nitriding gas into the furnace body 10. Hereinafter, the nitriding gas supply line 30 and the inert gas supply line 31 are simply called the gas supply lines 30 and 31. The lower portion of the nitriding furnace 1 is provided with a gas exhaust line 32 for discharging air or the atmosphere gas from the furnace body 10.

[0031] An upper portion of the retort 11 has an opening flange 14, in which a cover 15 is detachably provided. The temperature control unit 4 controls the heater 12 in response to the temperature detected by a temperature sensor 40 placed in the furnace body 10. The gas control unit 3 controls the introduction and discharge of the gas, the combination and exchange of the gases, the flow rate and pressure of the gas, and the like.

[0032] Examples of the object W to be nitrided include an austenitic stainless steel (SUS304), a martensitic stainless steel (SUS420J2), and a hot work tool steel (SKD61). Before nitriding, the object W is placed on a mount 17 provided in the retort 11 and then the cover 15 is attached to the retort 11 to seal it.

[0033]FIGS. 2 and 3 are schematic diagrams showing first and second examples of the vibration-applying means 2, respectively.

[0034] Referring to FIG. 2A, the first example of the vibration-applying means 2 is described. A passage pipe 24 is attached to the upper portion of the nitriding furnace 1. A nitriding gas introducing pipe 30, which serves as the nitriding gas supply line 30, is connected to an upper side 24 a of the passage pipe 24. A cylindrical agitator 20 is inserted and freely slidably provided in a lower portion of the passage pipe 24. The agitator 20 is suspended by a supporting rod 23 from a vibration transmitting plate 27 c outside the furnace 1. The agitator 20 has an outlet to the furnace 1 space, and a diaphragm 21 is attached to an outlet portion of the agitator 20. Numeral 25 represents a motor for applying vibration to the agitator 20. The motor 25 is fixed onto a casing 16, and as shown in FIG. 2B, its rotation is converted into up-and-down movements of the supporting rod 23 through a cam 27 a and a cam-follower 27 b, so that vibration is transmitted to the agitator 20.

[0035] The nitriding gas supplied from the nitriding gas introducing pipe 30 flows from the upper side 24 a to the lower side in the passage pipe 24 and collides against the agitator 20 which is vibrating at a high speed in up-and-down directions. By such a mechanism, vibration energy is applied to the nitriding gas, which involves a vibration-shock wave and is discharged into the furnace 1 through an outlet hole 22 formed in the agitator 20. At the moment, the nitriding gas also collides against the diaphragm 21 vibrating at a high speed to absorb additional vibration energy. The nitriding gas with the additional vibration energy is discharged into the nitriding furnace 1 to chemically react with the object W. In addition, a flexible sealing mechanism (such as a bellows mechanism) is provided at the upper portion of the passage pipe 24 to ensure the sealing between the passage pipe 24 and the supporting rod 23, and sleeve bearings are provided at upper and central portions of the supporting rod 23 to improve the stability and durability under the vibration.

[0036] Referring to FIG. 2B, the structure of the vibration-applying means 2 is described. In the drawing, numeral 26 represents an output shaft of the motor 25. Upon rotation of the shaft 26, the cam 27 a attached to it rotates in the direction indicated with arrow a and comes into contact with the cam-follower 27 b having a corrugated shape and placed on the plate 27 c, so that the cam 27 a allows the cam-follower 27 b to vibrate in the up-and-down directions indicated with arrow b together with the plate 27 c. The plate 27 c holding the cam-follower 27 b is movable in the up-and-down directions and supported by a plurality of supporting columns 28 b, which are firmly planted on the top of the body of the nitriding furnace 1, through coiled springs 28 a. To the plate 27 c is attached the upper end of the supporting rod 23 suspending the agitator 20. By this mechanism, the running torque of the motor shaft 26 is transmitted through the cam 27 a and the cam-follower 27 b to the plate 27 c in the form of vertical vibration of the plate 27 c, which results in vertical vibration of the agitator 20 and the diaphragm 21 through the supporting rod 23.

[0037] Referring to FIG. 3, the second example of the vibration-applying means 2 is described. This example of the vibration-applying means 2 also has a motor 25, an output shaft 26, a cam 27 a, a cam-follower 27 b, a plate 27 c, a coiled spring 28 a, and a supporting column 28 b in the same configuration as those of the first example, and the description thereof is omitted.

[0038] The difference between this example and the first example is that a passage pipe 24 is directly attached to the plate 27 c, and a diaphragm 21 is attached to a lower end of the passage pipe 24 so that the passage pipe 24 itself, through which the nitriding gas passes, is allowed to vertically vibrate for vibration of the diaphragm 21. This example is free of the agitator 20. A flexible sealing mechanism is provided at an upper portion of the passage pipe 24 to ensure the sealing between the passage pipe 24 and the top side of the nitriding furnace 1. A sleeve bearing is also provided at an upper portion of the supporting rod 23 to improve the stability and durability under the vibration. In this example, the passage pipe 24 is allowed to vibrate at an amplitude raging from about 1 to 10 mm and at a vibration frequency ranging from about 400 to 5,000 vibrations per minute (vpm) depending on the type of the object to be nitrided.

[0039] In the first example, the agitator 20 has the outlet hole 22. Alternatively, in the second example, a lower end portion of the passage pipe 24 has a plurality of outlet holes 24 b from which the nitriding gas is discharged through the passage pipe 24. The discharged nitriding gas is allowed to collide against the diaphragm 21 vibrating at a high speed so that the vibration energy is applied to the nitriding gas, which involves a vibration-shock wave together with the atmosphere gas in the retort 11 and reacts with the object W.

[0040]FIG. 4 is a diagram showing the gas flow. Referring to FIG. 4, the gas flow in the nitriding process is described in detail.

[0041] First, a supply valve V1 for supplying the gas into the retort 11 and an exhaust valve V2 for discharging the gas from the retort 11 are shut. An exhaust valve V3 is then opened, and a vacuum pump VP is run to evacuate air from the retort 11. After the evacuation, the supply valve V1 is opened to introduce the nitriding gas (NH₃) into the retort 11. If necessary, an inert gas (such as N₂) may be introduced before or together with the introduction of the nitriding gas.

[0042] After the retort 11 is filled up with an atmosphere gas containing the nitriding gas, an exhaust valve V4 is opened and the pressure of the atmosphere gas in the retort 11 is controlled with a mass flow controller MC or a pressure regulator PR. The pressure of the atmosphere gas is determined depending on the shape or material of the object W, or the hardness requirement for the nitrided layer.

[0043] After the pressure of the atmosphere gas becomes stable in the retort 11, the temperature inside the nitriding furnace 1 is elevated with the heater 12 (see FIG. 1). The temperature inside the nitriding furnace 1 is from 300 to 600° C. (depending on the shape or material of the object W, or the hardness requirement for the nitrided layer). In this example, a sufficient effect was obtained at a lower temperature (about 350° C.) than that of a conventional nitriding process (about 550° C.).

[0044] Vibration is then applied to the nitriding gas with the vibration-applying means 2, while the nitriding gas is introduced into the retort 11 for a certain time period. After the conclusion of the nitriding, the heater 12 is turned off and the object W is allowed to cool before taken out.

[0045] In this example, the vibration-applying means 2 is placed on the upper side of the nitriding furnace 1. Alternatively, such means may be placed on the lower side. If the nitriding furnace 1 holds a number of small objects W, the vibration-applying means 2 may be arranged at each of the upper and lower sides of the nitriding furnace 1 so that all the objects W can effectively be nitrided.

[0046] In addition, the object W may be rotated, while vibration is applied to the nitriding gas-containing atmosphere gas being supplied to the nitriding furnace 1. Such a process can reduce unevenness in nitriding the object W and provide uniform nitriding over the entire surface.

[0047] Nitriding processes were carried out according to the present invention and a conventional technique under the same conditions of temperature and time. The advantage of the present invention is examined from the resulting data.

[0048] FIGS. 5 to 7 show hardness of the object with respect to distances from the surface of the object. Parts A and B are a table and a graph each showing, in comparison, data obtained by the process according the present invention and the conventional process.

[0049] Referring to FIG. 5, the experimental data were obtained by nitriding austenitic stainless steel (SUS304) objects. The drawing shows that a rigid nitrided layer about 10 μm in thickness is formed in the surface of the object according to the present invention, but only a trace of nitrided layer is formed by the conventional process.

[0050] Referring to FIG. 6, the experimental data were obtained by nitriding martensitic stainless steel (SUS420J2) objects. The drawing shows that a rigid nitrided layer 15 to 20 μm in thickness is formed in the surface of the object according to the present invention, but a rigid nitrided layer only less than 5 μm in thickness is formed by the conventional process.

[0051] Referring to FIG. 7, the experimental data were obtained by nitriding hot work tool steel (SKD61) objects. The drawing shows that a rigid nitrided layer 10 to 15 μm in thickness is formed in the surface of the object according to the present invention, but only a trace of nitrided layer is formed by the conventional process.

[0052]FIG. 8 is a micrograph (×400) showing a surface side section of the martensitic stainless steel (SUS420J2) object, which was nitrided according to the present invention. FIG. 9 is a micrograph (×400) showing a surface side section of the martensitic stainless steel (SUS420J2) object, which was nitrided by the conventional process. The drawings show that a nitrided layer about 17 μm in thickness is formed in the surface of the object according to the present invention, but a nitrided layer only about 4 μm in thickness is formed by the conventional process.

[0053] From the resulting data, it has been confirmed that according to the present invention, the nitriding time can significantly be reduced under the same temperature conditions as those of the conventional gas nitriding method. According to the present invention, a sufficient nitriding effect was also obtained at a low temperature on objects each having a complicated shape, a small hole, a deep hole, or an unpierced hole.

[0054] In the above experimental examples, the diaphragm 21 was allowed to vibrate at a vibration frequency of about 1500 to 3500 vibrations per minute. At such a vibration frequency, a sufficient effect was confirmed. According to the results obtained by additional experiments, the nitriding effect has also been confirmed at a vibration frequency of the diaphragm 21 ranging from 400 to 5000 vibrations per minute.

[0055] In the above description, the dynamic nitriding method and apparatus according to the present invention are exemplified by the gas vibration type nitriding method and apparatus. In such examples, the vibration-applying means 2 is placed in the nitriding gas supply line 30, and the nitriding gas being provided with vibration is introduced into the nitriding furnace 1 so that the atmosphere gas in the furnace 1 is allowed to vibrate while the fluid catalytic reaction with the object surface is allowed to proceed. The experimental data and the photographs of the structures obtained by the examples are also shown. However, such examples are not intended to be limiting upon the scope of the invention.

[0056] For example, the vibration-applying means 2 may be provided in the inert gas supply line 31 so as to allow the nitriding gas in the furnace 1 to vibrate. This method may be combined with the above-described method so that the nitriding effect can further be improved.

[0057] In place of the vibration-applying means 2 provided in the gas supply lines 30 and 31, the diaphragm 21 or the like may be placed in the vicinity of the work (object to be nitrided) W in the retort 11. The diaphragm 21 may be allowed to vibrate through electromagnetic type or mechanical type vibration-applying means so as to provide the atmosphere gas with a vibration-shock wave in the vicinity of the object W. Such a mechanism can produce a similar effect and, of course, may be combined with the above-described mechanism.

[0058] Generally, atomic nitrogen from the nitriding gas greatly contributes to the nitriding reaction and therefore the vibration directly applied to the nitriding gas should be most effective.

[0059] In the dynamic nitriding method according to the present invention, the step of applying vibration to the nitriding gas or the atmosphere gas may be replaced by the step of applying vibration energy to the object W itself. By such a step, the activation of the nitriding reaction of the object W with the nitriding gas can be facilitated so that a similar effect can be produced. Specifically, a small object W may be placed on a vibrating mount and allowed to vibrate under the nitriding gas atmosphere in the retort 11. The vibration of the object W itself may also be combined with the vibration of the atmosphere gas. In such a case, the nitriding would further be facilitated, and the nitriding time can be reduced.

[0060] Besides the mechanical vibration, the vibration-applying means 2 may comprise means for applying ultrasonic vibration to the nitriding gas-containing atmosphere gas or means for generating low frequency molecular vibration. Such means can complementarily be combined with the mechanical vibration means so that the effect can further be enhanced.

[0061] The above-described methods, including the method comprising the step of applying vibration or a shock wave to the gas in nitriding, the method comprising the step of allowing the object to vibrate in nitriding, and the method in which both of the above-mentioned steps are combined, are collectively called the dynamic nitriding method in contrast to the conventional static nitriding method. The dynamic nitriding method is characterized in that high rate nitriding can be achieved at a low temperature.

INDUSTRIAL APPLICABILITY

[0062] In the present invention, the nitriding reaction is allowed to occur under the environmental conditions including the artificial application of vibration. In such conditions, the catalytic effect of the steel surface can be enhanced and the thermal decomposition reaction of the nitriding gas can be accelerated. In the present method, the dissociation of nitrogen atoms from ammonia gas is so facilitated that much nascent hydrogen can be generated and such hydrogen atoms can provide a stronger reducing action. As a result, a more stable nitrided layer can be obtained together with an etching effect on the steel surface.

[0063] As described above, according to the present invention, effective nitriding can be performed on nitriding-resistant materials such as austenitic stainless steels and martensitic stainless steels, and complex-shaped objects having edges, small holes, or deep holes. The present invention is also advantageously applied to frequently used materials such as hot work tool steels, because nitriding can be performed at a lower temperature for a shorter time period with the embrittled layer (white layer) significantly reduced and with less harmful effect on the internal structure of the object than the conventional nitriding method.

[0064] According to the present invention, the process at a lower temperature for a shorter time period can reduce the usage of the nitriding gas and nitriding furnace heating energy, improve the working environment, and therefore be economically and environmentally advantageous. 

1. A method for dynamically nitriding an object, comprising the steps of: nitriding said object held under a nitriding gas atmosphere in a sealed furnace; and applying vibration energy to one or both of said nitriding gas and said object to facilitate nitriding.
 2. The method according to claims 1, wherein said vibration energy is so selected as to generate vibration at a vibration frequency of 400 to 5000 vibrations per minute.
 3. A gas vibration type nitriding apparatus, comprising: a nitriding furnace for holding an object to be nitrided in a sealed manner; means for supplying a nitriding gas to said nitriding furnace; and means for applying vibration to an atmosphere gas in said nitriding furnace, wherein nitriding will be facilitated by allowing said atmosphere gas to vibrate.
 4. A gas vibration type nitriding apparatus, comprising: a nitriding furnace for holding an object to be nitrided in a sealed manner; gas supply lines for supplying a nitriding gas and/or one or more atmosphere gases other than said nitriding gas to said nitriding furnace; and means for applying vibration to one or more gases in some or all of said gas supply lines, wherein nitriding will be facilitated by atmosphere gas vibration.
 5. The gas vibration type nitriding apparatus according to claim 3 or 4, wherein said means for applying vibration is a fast vibrating diaphragm, and one or more gases being supplied to said furnace are allowed to collide against said vibrating diaphragm to apply vibration to atmosphere gas in said furnace. 